Copper oxides supported on spinel oxides as catalysts for low temperature direct NOx decomposition

ABSTRACT

Active catalysts for the treatment of a low temperature exhaust gas stream are provided containing copper oxides dispersed on a spinel oxide for the direct, lean removal of nitrogen oxides from the exhaust gas stream. The low temperature, direct decomposition is accomplished without the need of a reductant molecule. In one example, CuO x  may be dispersed as a monolayer on a metal oxide support, such as Co 3 O 4  spinel oxide, synthesized using an incipient wetness impregnation technique. The CuO x /Co 3 O 4  catalyst system converts nitric oxide to nitrogen gas with high product specificity, avoiding the production of a significant concentration of the undesirable N 2 O product.

TECHNICAL FIELD

The present disclosure generally relates to catalysts for treatment ofan exhaust gas stream and, more particularly, to catalysts containingcopper oxides on a spinel for removal of nitrogen oxides from a lowtemperature exhaust gas stream as generated by an internal combustionengine, or the like.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it may be described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presenttechnology.

Catalysts effective at removing NOx from exhaust emissions are desirablein order to protect the environment and to comport with regulationsdirected to that purpose. It is preferable that such catalysts convertNOx to inert nitrogen gas, instead of converting NOx to othernitrogen-containing compounds. Catalysts that are effective at lowtemperature may have additional utility for vehicles.

Increasingly stringent NOx emission and fuel economy requirements forvehicles and automobile engines will require catalytic NOx abatementtechnologies that are effective under lean-burn conditions. Direct NOxdecomposition to N₂ and O₂ is an attractive alternative to NOx traps andselective catalytic reduction (SCR) for this application, as NOx trapsand SCR processes are highly dependent on reductants (such as unburnedhydrocarbons or CO) to mitigate NOx. The development of an effectivecatalyst for direct NOx decomposition would eliminate the use ofreducing agents, simplifying the NOx removal process, and thereforedecreasing the fuel efficiency cost of NOx abatement.

However, most catalysts active for direct NOx decomposition are onlyefficient at high temperatures (i.e., greater than about 600° C.), whichseverely limits a practical application for a vehicle exhaust gasstream. The most well-known low temperature (i.e., less than about 500°C.) direct NOx decomposition catalysts include Cu-ZSM5, K/Co₃O₄,Na/Co₃O₄, CuO, and Ag/Co₃O₄. However, low temperature activity andselectivity to N₂ for all of these catalysts is not sufficient forpractical application, and more advancements are needed. Advancements indirect NOx decomposition catalysis based on structure activityrelationships are lacking, and methodology to improve the performance ofspecific catalyst systems is needed.

Accordingly, it would be desirable to provide a catalyst for the removalof NOx from exhaust gas, that is effective at low temperature and thathas high N₂ product specificity.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

In various aspects, the present teachings provide a catalyst system forthe direct decomposition removal of NO_(x) from an exhaust gas stream.In various aspects, the exhaust stream is provided at a temperature ofless than or equal to about 500° C. The catalyst system includes a metaloxide support, such as a Co₃O₄ spinel oxide. A monolayer of CuO_(x) isdisposed on a surface of the Co₃O₄ spinel oxide, and configured tocatalyze a reduction of the NOx to generate N₂ without the presence of areductant.

In other aspects, the present teachings provide a catalytic converterfor the direct decomposition removal of NO_(x) from an exhaust gasstream flowing at a temperature of less than or equal to about 500° C.In various aspects, the catalytic converter includes an inlet configuredto receive the exhaust gas stream into an enclosure, and an outletconfigured to allow the exhaust gas stream to exit the enclosure. Acatalyst system may be contained inside the enclosure. The catalystsystem may include a monolayer of CuO_(x) on a metal oxide support thatis configured to catalyze a reduction of the NOx to generate N₂ withoutthe presence of a reductant. The metal oxide support may include a Co₃O₄spinel oxide. The monolayer of CuO_(x) may be supported on the Co₃O₄spinel oxide using incipient wetness impregnation techniques leading tothe formation of discrete island regions of CuO_(x) on a surface of thespinel oxide.

In still further aspects, the present teachings provide methods for thedirect decomposition removal of NO_(x) from a low temperature exhaustgas stream. In various implementations, the methods may include flowingthe exhaust gas stream through an enclosure with a catalyst system, andexposing the exhaust gas stream to a copper oxide supported on a metaloxide support, for example, CuO_(x)/Co₃O₄. The methods includecatalyzing a reduction of the NOx to generate N₂ without the presence ofa reductant.

Further areas of applicability and various methods of enhancing theabove coupling technology will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present teachings will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a schematic illustration showing a flow process forCuO_(x)/Co₃O₄ synthesis by incipient wetness impregnation;

FIG. 2 is an x-ray diffraction (XRD) pattern of 6.7 CuO_(x)/Co₃O₄ ascompared to Co₃O₄;

FIG. 3 is a scanning transmission electron microscope (STEM) view of10.3 CuO_(x)/Co₃O₄ indicating a presence of CuO_(x) islands on the Co₃O₄surface in excess of a monolayer;

FIG. 4 is an XPS plot indicating the presence of Cu²⁺ oxide;

FIG. 5 provides an XPS peak area ratio of Cu/Co versus Cu loading,indicating a CuO_(x) monolayer formation between about 6.7 and 7.6Cu/nm²;

FIG. 6 is a schematic illustration of a sub-monolayer, monolayer, andabove-monolayer coverage of supported metal oxide catalyst;

FIG. 7 is a schematic illustration of the direct NOx decompositionreaction process;

FIG. 8 is a plot showing a comparison of the NOx selectivity to N₂ (as a%) between CuO_(x)/Co₃O₄ and Cu-ZSM5;

FIG. 9 is a plot showing a comparison of M/Z 44 (N₂O) relative abundanceover CuO_(x)/Co₃O₄ and Cu-ZSM5 during direct NO decomposition;

FIG. 10 illustrates NO decomposition mass activity to N₂ overCuO_(x)/Co₃O₄ as a function of Cu surface density at 400° C. and 450°C.;

FIG. 11 illustrates surface area-normalized direct NO decompositionactivity over CuO_(x)/Co₃O₄ of several CuO_(x) surface densities andCu-ZSM5 at various temperatures;

FIG. 12 illustrates NO adsorption capacity at 100° C. of Co₃O₄,CuO_(x)/Co₃O₄, and CuO;

FIG. 13 illustrates the M/Z 30 (NO) profile between 225° C. and 500° C.during NO-TPD over Co₃O₄, CuO, and 6.7 CuO_(x)/Co₃O₄.

It should be noted that the figures set forth herein are intended toexemplify the general characteristics of the methods, algorithms, anddevices among those of the present technology, for the purpose of thedescription of certain aspects. These figures may not precisely reflectthe characteristics of any given aspect, and are not necessarilyintended to define or limit specific embodiments within the scope ofthis technology. Further, certain aspects may incorporate features froma combination of figures.

DETAILED DESCRIPTION

The present teachings provide an active catalyst for the treatment of alow temperature exhaust gas stream. The catalyst includes copper oxidesdispersed on a metal oxide support for the direct, lean removal ofnitrogen oxides from the exhaust gas stream. The low temperature, directdecomposition is accomplished without the need of a reductant (i.e., H₂,CO, C₃H₆ or other hydrocarbons, and/or soot), thereby improving fuelefficiency. Direct decomposition, as discussed herein, refers tocatalytic transformation of nitrogen oxides to elemental nitrogen andoxygen. This differs, for example, from catalytic reduction of nitrogenoxides to ammonia and water. In one example, CuO_(x) may be dispersed asa monolayer on a metal oxide support, such as Co₃O₄ spinel oxide,synthesized using an incipient wetness impregnation technique. TheCuO_(x)/Co₃O₄ catalyst system converts nitric oxide to nitrogen gas withhigh product specificity, all while avoiding the production of asignificant concentration of the undesirable N₂O product. In variouspreferred aspects, the CuO_(x)/Co₃O₄ catalyst may be operable at exhaustgas/stream temperatures lower than about 500° C., lower than about 450°C., lower than about 400° C., lower than about 350° C., lower than about325° C., and even lower than or at about 300° C.

The presently disclosed catalyst system includes methods for dispersingcopper oxide on a metal oxide support, specifically a spinel oxide withknown N₂O decomposition activity (i.e., Co₃O₄), via incipient wetnessimpregnation. This method particularly provides for improved total yieldof product N₂ and product selectivity to N₂ (versus undesired N₂O and/orNO₂ products) during low temperature direct NOx decomposition ascompared to either the bare Co₃O₄ spinel oxide support only or thepreviously known most active direct NO decomposition catalyst: theion-exchanged Cu-zeolite Cu-ZSM5. It should be understood that althoughthe total percentage of NO conversion may be higher over Cu-ZSM5, muchof the NO is converted to the highly undesirable product and potentgreenhouse gas, N₂O. Because of the high selectivity to N₂ for thepresent teachings, the undesirable N₂O product is not produced in asignificant quantity during the direct NO decomposition over Co₃O₄spinel-supported copper oxide. Additionally, it has been discoveredthat, on a per surface area basis, CuO_(x)/Co₃O₄ is more active thanCu-ZSM5.

As detailed herein, the present teachings not only include thedevelopment of the catalyst system, but also the utilization of thecatalyst system with exhaust gas streams, particularly with catalyticconverters for vehicles, automobiles, and the like, as well as includingmethods of synthesizing the CuO_(x) supported in the spinel oxide.

NOx decomposition over Cu-ZSM5 or Cu—Co/Al₂O₃ catalysts, which may beconsidered by some to be the closest technology related to the presentteachings, has poor selectivity to N₂ as a result of significant N₂Oproduction. This product is undesirable as N₂O is a highly potentgreenhouse gas, several hundred times more potent than CO₂.Specifically, the Cu—Co/Al₂O₃ catalyst is oxidized during operation,quickly losing activity, and is, therefore, not suitable for catalyticapplications requiring long lifetimes. Alternatively, the CuO_(x)/Co₃O₄as disclosed in the present technology displays good activity to N₂production even after 2 hours on stream at various temperatures.Furthermore, the activity of the spinel supported CuO_(x) can beoptimized by formation of a CuO_(x) monolayer on the spinel surface, andthe resulting catalyst system has a greater NO decomposition activitythan Cu-ZSM5 on a per surface area basis.

The catalyst system of the present disclosure can be used in a chamberor an enclosure, such as a catalytic converter, having an inlet and anoutlet. As is commonly known to those of ordinary skill in the art, sucha chamber or enclosure can be configured to receive an exhaust gasstream through the inlet and to exit the exhaust gas stream through theoutlet, such that the exhaust gas stream has a particular or definedflow direction.

EXAMPLES

Various aspects of the present disclosure are further illustrated withrespect to the following Examples. It is to be understood that theseExamples are provided to illustrate specific embodiments of the presentdisclosure and should not be construed as limiting the scope of thepresent disclosure in or to any particular aspect.

Synthesis and Material Characterization

FIG. 1 is a schematic illustration showing an exemplary flow process forCuO_(x)/Co₃O₄ synthesis by incipient wetness impregnation (IWI)techniques. The IWI of a copper nitrate aqueous solution is prepared ofa volumetric quantity equal to the total pore volume of the commercialCo₃O₄, as determined by N₂ Physisorption at P/P₀≈0.98. As shown in FIG.1, the solution is heated at about 100° C. for about 12 hours to removewater, and subsequently heated at about 450° C. to remove NO₂ and O₂.The resulting material is subsequently dried and calcined. Table 1,reproduced below, summarizes data from a total of 10 samples, withtheoretical and empirical reagent quantities listed in order to achieveCu surface densities ranging from approximately 0.9 to 10.3 Cu atoms persquared nanometer (Cu/nm²), as determined via ICP. In various preferredembodiments, the CuO_(x) is provided as a monolayer that includes atwo-dimensional, non-crystalline structure. The monolayer may preferablybe provided with a surface density of Cu at an empirical loading levelof between about 5.9 and about 7.6 Cu atoms per square nanometer(Cu/nm²), or about 6.7 Cu/nm², as determined by ICP techniques.

TABLE 1 Theoretical and Empirical Reagent Quantities to SynthesizeCuO_(x)/Co₃O₄ with various Cu surface densities Theoretical EmpiricalTheoretical Empirical Nominal Empirical Empircal Mass Mass Mass MassTheoretical Empirical Nominal Cu Cu Cu Cu Commercial CommercialCu(NO₃)₂*2.5H₂O Cu(NO₃)₂*2.5H₂O Volume Volume Loading Loading LoadingLoading Co₃O₄ (g) Co₃O₄ (g) (mg) (mg) H₂O (μl) H₂O (μl) (Cu/nm²) (wt %)(Cu/nm²) (wt %) 3 3.001 40.3 40.3 0.677 0.677 1 0.37 0.9 0.33 3 3.00280.6 81.0 0.668 0.667 2 0.73 1.9 0.69 3 3.000 120.9 120.8 0.659 0.659 31.08 2.8 1.02 3 3.002 161.2 161.6 0.649 0.649 4 1.45 3.8 1.38 3 2.999222 222.2 0.635 0.635 5.5 1.98 5.1 1.83 3 2.998 251.6 251.4 0.628 0.6276.25 2.24 5.9 2.1 3 2.997 282 281.0 0.621 0.620 7 2.5 6.7 2.33 3 3.001322.3 322.3 0.612 0.612 8 2.85 7.6 2.71 3 3.002 362.6 362.2 0.603 0.6039 3.2 8.3 2.97 3 3.000 443.3 443.4 0.584 0.584 11 3.88 10.3 3.62

FIG. 2 is an x-ray diffraction (XRD) pattern of 6.7 CuO_(x)/Co₃O₄ ascompared to Co₃O₄ and provides evidence to show the material phase. Asshown in FIG. 2, the only phase detected via the powder XRD of the bestperforming material, 6.7 CuO_(x)/Co₃O₄, is attributed to the Co₃O₄spinel support.

In order to provide evidence of the material structure, FIG. 3 isprovided showing a scanning transmission electron microscope (STEM) viewof 10.3 CuO_(x)/Co₃O₄ FIG. 3 indicates a presence of the Cu speciesexisting as CuO_(x) in the form of discrete island regions on the Co₃O₄spinel oxide surface. In various aspects, the monolayer of CuO_(x)comprises a two-dimensional, non-crystalline, and/or polymeric-likestructure. In various aspects, the Co₃O₄ spinel oxide may be present ina nanoparticle form, for example, having an average diameter of fromabout 2 to about 100 nm, or from about 2 to about 50 nm.

In order to further provide evidence of the material structure, FIG. 4is provided, illustrating an annotated XPS plot. As indicated, the XPSdata clearly specifies the presence of Cu²⁺ oxide in the as-preparedmaterial. Still further, FIG. 5 provides an XPS peak area ratio of Cu/Coversus Cu loading. A review of FIG. 5, showing the deviation fromlinearity between the Cu surface density (x-axis) and the areal ratio ofCu/Co (y-axis), indicates a CuO_(x) monolayer formation at a particularCu loading of between about 6.7 and 7.6 Cu/nm².

FIG. 6 is a schematic illustration of a sub-monolayer, monolayer, andabove-monolayer coverage of supported metal oxide catalyst. Thisschematically shows the type of oxide coverage based on the surfacedensity of CuO_(x). At an empirical Cu loading below about 6.7 Cu/nm²,the CuO_(x) species are in a sub-monolayer regime and are highlydispersed, non-crystalline species. At an empirical Cu loading fromabout 6.7 to 7.6 Cu/nm², the CuO_(x) creates a monolayer on the Co₃O₄that is a two-dimensional, non-crystalline, polymeric-like structure. Atan empirical Cu loading above about 7.6 Cu/nm², the monolayer coverageis exceeded and three-dimensional, bulk-like crystals form and grow.

Performance Evaluation

FIG. 7 is an exemplary schematic illustration of the direct NOxdecomposition reaction process. The materials of the above examples areevaluated using a micro reactor system (Micromeritics ParticulateSystems PID Microactivity Reactor), equipped with a quartz plug flowreactor and coupled with a mass spectrometer (MKS Cirrus-2). The NOconcentration is tracked by the detector signal for M/Z=30. In order tomonitor products, the intensities at M/Z=28, 16 & 32, 44, and 46 aretracked for N₂, O₂, N₂O, and NO₂, respectively. Approximately 550 mg ofthe CuOx/Co₃O₄, or 300 mg of Cu-ZSM5, is placed between a bed of quartzwool to maintain a 1 cm bed length of catalyst, for total gas hourlyspace velocity (GHSV) of about 2,100 h⁻¹. The samples are pretreated to500° C. at a ramp rate of 10° C./min in 27.8 sccm of 10% O₂/He, held fora total of 50 minutes, and cooled to the initial reaction temperature(T₁, in most cases is 300° C., 350° C., or 400° C.).

In order to determine the mass spectrometer signal corresponding to 100%conversion of NO, 27.8 sccm of UHP He is flown over the bypass. Then, areaction mixture containing approximately 1% NOx/He (≈9500 ppm NO,≈75-100 ppm NO₂, ≈25-45 ppm N₂O) is flown at 27.8 sccm for 75 minutes todetermine the mass spectrometer signal corresponding to 0% conversion ofNO. Next, the flow is stabilized over the catalyst, and the reactionmixture at T₁ is conducted for 2 hours. After reaction at T₁, thereactor is purged for about 15 minutes with UHP He, and then ramped tothe next reaction temperature, T₂ (for example, 325° C., 375° C., or450° C., depending on T₁), and performance is evaluated for two hours.If desired, performance at a third reaction temperature, T₃, isevaluated for two hours, prior to which the reactor is purged for about15 minutes in UHP He, and then ramped to T₃ at 10° C./min (for example,T₃ could be either 350° C. or 500° C., based on T₂). To determine thetotal N₂ production, a calibration gas consisting of 1137 ppm N₂ in a Hebalance is utilized to calibrate the M/Z=28 response by creating acalibration curve. The calibration curve is utilized to calculate aquantified N₂ production.

The total NO conversion (determined by averaging the mass spectrometersignal within the final 3 minutes on stream at each steady-statetemperature), is utilized to calculate the rate of NO consumption for N₂production. The rates are then used to subsequently calculate themass-normalized activity to N₂ and Cu molar activity to N₂. While thetotal NO conversion of Cu-ZSM5 is higher than CuOx/Co₃O₄ at temperaturesof from about 300° C. to 350° C., with similar mass-normalized activityto N₂, it can be seen that the N₂ selectivity was greater for theCuOx/Co₃O₄ from about 300° C. to 350° C. FIG. 8 is a plot showing acomparison of the NOx selectivity to N₂ (as a %) between CuO_(x)/Co₃O₄and Cu-ZSM5. In various aspects, the present teachings can be configuredto flow the exhaust gas stream through the catalyst system at atemperature at or less than about 300° C. and obtain an NOx selectivityto N₂ greater than about 15%. In various other aspects, the presentteachings can be configured to flow the exhaust gas stream through thecatalyst system at a temperature at or less than about 350° C. andobtain an NOx selectivity to N₂ greater than about 20%.

Furthermore, a significant portion of the NO conversion over Cu-ZSM5catalyst is attributed to production of N₂O, which is significantlygreater than that of CuOx/Co₃O₄. FIG. 9 is a plot showing a comparisonof M/Z 44 (N₂O) relative abundance over CuO_(x)/Co₃O₄ and Cu-ZSM5 duringdirect NO decomposition;

The total mass-normalized activity to N₂ reaches a maximum for theCuOx/Co₃O₄ system around the onset of monolayer coverage at 400° C. and450° C., suggesting that the Cu—O—Co interface plays a key role in thedirect NO decomposition performance. FIG. 10 illustrates NOdecomposition mass activity to N₂ over CuO_(x)/Co₃O₄ as a function of Cusurface density at 400° C. and 450° C.

FIG. 11 illustrates surface-area normalized direct NO decompositionactivity over CuO_(x)/Co₃O₄ of several CuO_(x) surface densities andCu-ZSM5 at various temperatures. As shown, FIG. 11 indicates that thetotal surface-area normalized activity to N₂ is up to 4× higher forCuO_(x)/Co₃O₄ (BET Specific Surface Area≈26 m²/g) as compared to Cu-ZSM5(BET Specific Surface Area≈317 m²/g) at 400° C. The CuO_(x)/Co₃O₄maintains the advantage at 450° C. and 500° C. as well.

In an effort to begin to understand the mechanism for the improved NOdecomposition activity over the CuOx/Co₃O₄, NO adsorption/desorptionperformance for CuOx/Co₃O₄ is compared to that of the individualcomponents (CuO, Co₃O₄). For example, the NO adsorption/desorption ismonitored using a Thermogravimetric Analyzer Coupled with online MassSpectrometer (Netzsch Jupiter STA449 F1 and Aëolos QMS4030DA00.000-00).After pretreatment to 500° C. in 10% O₂/Ar for 50 minutes (similar tothe reaction), the temperature is cooled to 100° C. NO is allowed topass for about 3 hours. The difference in mass both prior to NOadsorption and after NO adsorption is measured and utilized to determinethe total NO adsorption capacity of the materials in μmol per gramsample. FIG. 12 illustrates NO adsorption capacity at 100° C. of Co₃O₄,CuO_(x)/Co₃O₄, and CuO. As shown, the thermogravimetric results revealan increased NO adsorption capacity for CuO_(x)/Co₃O₄ as compared to theindividual Co₃O₄ or CuO components. In various aspects, the presenttechnology can provide a catalytic converter configured to exhibit an NOadsorption capacity of greater than about 200 μmol NO/g, significantlyhigher than that of CuO_(x) and/or Co₃O₄ individually.

After adsorption, the NO gas is turned off, and the materials are rampedin Ar from 100° C. to 500° C. at 10° C./min while monitoring thedesorption of NO via the mass spectrometer signal at M/Z=30. FIG. 13illustrates the M/Z 30 (NO) profile between 225° C. and 500° C. duringNO-TPD over Co₃O₄, CuO, and 6.7 CuO_(x)/Co₃O₄. Notably, the CuOindividual component does not desorb significant amounts of NO from 100°C. to 500° C., reaching a maximum rate at about 336° C. This relativelyhigh maximum desorption temperature indicates the NO surface species onCuO are bound too strongly and are too scarce (low NO adsorptioncapacity) to maintain good activity. Co₃O₄ reaches a maximum NOdesorption rate at about 286° C., releasing significant NO at relativelylow temperature. The relatively low maximum adsorption temperature onthe Co₃O₄ is averse to maintaining high activity as the NO surfacespecies have desorbed prior to achieving a high enough reactiontemperature. However, CuO_(x)/Co₃O₄ reaches a maximum NO desorption rateat about 314° C., indicating intermediate binding strength of theadsorbed NO surface species compared to the parent materials CuO andCo₃O₄.

The preceding description is merely illustrative in nature and is in noway intended to limit the disclosure, its application, or uses. As usedherein, the phrase at least one of A, B, and C should be construed tomean a logical (A or B or C), using a non-exclusive logical “or.” Itshould be understood that the various steps within a method may beexecuted in different order without altering the principles of thepresent disclosure. Disclosure of ranges includes disclosure of allranges and subdivided ranges within the entire range.

The headings (such as “Background” and “Summary”) and sub-headings usedherein are intended only for general organization of topics within thepresent disclosure, and are not intended to limit the disclosure of thetechnology or any aspect thereof. The recitation of multiple embodimentshaving stated features is not intended to exclude other embodimentshaving additional features, or other embodiments incorporating differentcombinations of the stated features.

As used herein, the terms “comprise” and “include” and their variantsare intended to be non-limiting, such that recitation of items insuccession or a list is not to the exclusion of other like items thatmay also be useful in the devices and methods of this technology.Similarly, the terms “can” and “may” and their variants are intended tobe non-limiting, such that recitation that an embodiment can or maycomprise certain elements or features does not exclude other embodimentsof the present technology that do not contain those elements orfeatures.

The broad teachings of the present disclosure can be implemented in avariety of forms. Therefore, while this disclosure includes particularexamples, the true scope of the disclosure should not be so limitedsince other modifications will become apparent to the skilledpractitioner upon a study of the specification and the following claims.Reference herein to one aspect, or various aspects means that aparticular feature, structure, or characteristic described in connectionwith an embodiment or particular system is included in at least oneembodiment or aspect. The appearances of the phrase “in one aspect” (orvariations thereof) are not necessarily referring to the same aspect orembodiment. It should be also understood that the various method stepsdiscussed herein do not have to be carried out in the same order asdepicted, and not each method step is required in each aspect orembodiment.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations should not beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. A catalyst system for the direct decompositionremoval of NOx from an exhaust gas stream provided at a temperature ofless than about 500° C., the catalyst system comprising: a Co₃O₄ spineloxide; and a plurality of discrete island regions of CuO_(x) supportedon a surface of the Co₃O₄ spinel oxide and configured to catalyze areduction of the NOx to generate N₂ without the presence of a reductant,wherein each of the plurality of the discrete island regions of CuOxcomprises a two-dimensional, non-crystalline structure provided with asurface density of Cu at an empirical loading level of between about 6.7and about 7.6 Cu atoms per square nanometer (Cu/nm²) as determined byICP techniques.
 2. The catalyst system according to claim 1, wherein thediscrete island regions of CuO_(x) supported on the Co₃O₄ spinel oxideare formed using incipient wetness impregnation techniques.
 3. Acatalytic converter for the direct decomposition removal of NOx from anexhaust gas stream flowing at a temperature of less than or equal toabout 500° C., the catalytic converter comprising: an inlet configuredto receive the exhaust gas stream into an enclosure; an outletconfigured to allow the exhaust gas stream to exit the enclosure; and acatalyst system contained inside the enclosure, the catalyst systemcomprising a plurality of discrete island regions of CuOx supported on asurface of a Co₃O₄ spinel oxide support that is configured to catalyze areduction of the NOx to generate N₂ without the presence of a reductant,wherein each of the plurality of the discrete island regions of CuOxcomprises a two-dimensional, non-crystalline structure provided with asurface density of Cu at an empirical loading level of between about 6.7and about 7.6 Cu atoms per square nanometer (Cu/nm²) as determined byICP techniques.
 4. The catalytic converter according to claim 3, whereinthe Co₃O₄ spinel oxide is in a nanoparticle form, having an averagediameter of from about 2 to about 100 nm.
 5. The catalytic converteraccording to claim 3, wherein the discrete island regions of CuOxsupported on the Co₃O₄ spinel oxide are formed using incipient wetnessimpregnation techniques.
 6. The catalytic converter according to claim3, wherein the catalyst system is provided with a surface density of Cuat an empirical loading level of about 6.7 Cu atoms per square nanometer(Cu/nm²) as determined by ICP techniques.
 7. The catalytic converteraccording to claim 3, configured to flow the exhaust gas stream throughthe catalyst system at a temperature at or less than about 350° C. andobtaining an NOx selectivity to N₂ greater than about 20%.
 8. Thecatalytic converter according to claim 3, configured to flow the exhaustgas stream through the catalyst system at a temperature at or less thanabout 300° C. and obtaining an NOx selectivity to N₂ greater than about15%.
 9. The catalytic converter according to claim 3, configured toexhibit an NO adsorption capacity of greater than about 200 μmol NO/g.10. A method for direct decomposition removal of NOx from a lowtemperature exhaust gas stream, the method comprising: flowing theexhaust gas stream through a catalyst system and exposing the exhaustgas stream to a plurality of discrete island regions of copper oxidesupported on a Co₃O₄ spinel oxide support, each of the plurality of thediscrete island regions of CuOx comprising a two-dimensional,non-crystalline structure provided with a surface density of Cu at anempirical loading level of between about 6.7 and about 7.6 Cu atoms persquare nanometer (Cu/nm²) as determined by ICP techniques; andcatalyzing a reduction of the NOx to generate N₂ without the presence ofa reductant.
 11. The method according to claim 10, wherein the CuOx issupported on the Co₃O₄ spinel oxide using incipient wetness impregnationtechniques.
 12. The method according to claim 10, comprising flowing theexhaust gas stream through the catalyst system at a temperature at orless than about 350° C. and obtaining an NOx selectivity to N₂ greaterthan about 20%.
 13. The method according to claim 10, comprising flowingthe exhaust gas stream through the catalyst system at a temperature ator less than about 300° C. and obtaining an NOx selectivity to N₂greater than about 15%.